Article including thermal barrier coated superalloy substrate

ABSTRACT

A ceramic thermal barrier coating layer for a superalloy article is caused to adhere to the superalloy article by applying platinum to the superalloy article and heat treating at a temperature of 1100° C. to 1200° C. for one hour. This causes aluminum to diffuse from the superalloy article into the platinum to form a platinum enriched outer layer which generally includes a platinum enriched gamma phase and a platinum enriched gamma prime phase. An alumina layer is formed between the platinum enriched outer layer and a ceramic coating. The platinum enriched gamma phase and the platinum enriched gamma prime phase in the outer layer reduces the migration of transition metal elements to the ceramic coating to enable a very pure alumina layer to be formed.

This is a Division of Application Ser. No. 08/574,900 filed Dec. 19,1995 now U.S. Pat. No. 5,667,663.

FIELD OF THE INVENTION

The present invention relates to a method of applying a thermal barriercoating to the surface of a superalloy article, e.g. a gas turbineengine turbine blade.

BACKGROUND

The constant demand for increased operating temperatures in gas turbineengines was initially met by air cooling of the turbine blades anddevelopment of superalloys from which to manufacture the blades, both ofwhich extended their service lives. Further temperature increasesnecessitated the development of ceramic coating materials with which toinsulate the turbine blades from the heat contained in the gasesdischarged from the combustion chamber and again turbine operating lifewas extended. However, the amount of life extension was limited becausethe coatings suffered from inadequate adhesion to the superalloysubstrates, one reason for this being the disparity of coefficients ofthermal expansion between the superalloy substrate and the ceramiccoating. Coating adhesion was improved by the development of varioustypes of aluminum-containing alloy bond coats which were thermallysprayed or otherwise applied to the superalloy substrate before theapplication of the ceramic coating. Such bond coats are typically of theso-called aluminide (diffusion) or "MCrAlY" types, where M signifies oneor more of cobalt, nickel and iron.

Use of bond coats has been successful in preventing extensive spallationof thermal barrier coatings during service, but localized spallation ofthe ceramic still occurs where the adhesion fails between the bond coatand the ceramic layer. This exposes the bond coat to the full heat ofthe combustion gases, leading to premature failure of the turbine blade.

The bond coats of the aluminide (diffusion) type are disclosed forexample in U.S. Pat. Nos. 4,880,614, 4,916,022 and 5,015,502. This typeof bond coat is produced by reacting aluminum with the superalloysubstrate to produce a diffusion aluminide bond coat. The aluminum isreacted with the superalloy substrate by any of the commerciallyavailable aluminizing processes using aluminum vapors or aluminum richalloy powder, for example pack aluminizing, chemical vapor deposition,sputtering, electrophoresis, etc. and is followed by diffusion heattreatment. These patents also disclose the use of platinum aluminidebond coats on the superalloy substrate.

The bond coats of the MCrAlY type are disclosed in the U.S. Pat. Nos.4,321,311, 4,401,697 and 4,405,659. This type of bond coat is producedby depositing a MCrAlY alloy onto the superalloy substrate.

Furthermore the three U.S. Pat. Nos. 4,880,614, 4,916,022 and 5,015,502mentioned above also disclose the use of an aluminide coating inconjunction with a MCrAlY coating as a bond coating. More specificallythey disclose that the substrate is first aluminized, as discussedabove, and then a MCrAlY coating is applied onto the aluminizedsuperalloy substrate.

Also International Patent application No. WO93/18199 discloses the useof an aluminide coating in conjunction with a MCrAlY coating as a bondcoating. More specifically it discloses that the superalloy has a MCrAlYcoating with an aluminide top coating or a MCrAlY coating with aplatinum aluminide top coating.

It is further known from U.S. Pat. No. 4,399,199 to provide aplatinum-group metal layer on a superalloy substrate as a bond coatingfor a ceramic thermal barrier coating. The platinum-group metal is heattreated at 700° C. to bond the platinum-group metal to the superalloysubstrate.

It is also known from U.S. Pat. No. 5,427,866 to provide aplatinum-group metal layer on a superalloy substrate as a bond coatingfor a ceramic thermal barrier coating. The platinum-group metal is heattreated at 980° C. to 1095° C. to form an interdiffusion region ofplatinum-group metal aluminide between the superalloy substrate and theplatinum-group metal.

A problem associated with the production of the platinum aluminide onthe superalloy substrate is that the use of the conventional aluminizingprocess, e.g. pack aluminizing, uses a pack containing aluminum oxidepowder and aluminum halide which produces aluminum vapors to react withplatinum deposited on the superalloy substrate. This pack also containsundesirable elements, or impurities, which also react with the platinumon the superalloy substrate leading to poor adhesion between theplatinum aluminide and the ceramic coating.

A problem associated with the production of the interdiffusion region ofplatinum aluminide between the platinum and the superalloy substrate isthat the bond coating is unstable leading to poor adhesion between theceramic coating and the bond coating.

SUMMARY OF THE INVENTION

Thus one object of the present invention is to provide a method ofapplying a thermal barrier coating to a superalloy substrate so as toachieve improved adhesion thereto.

Accordingly the present invention provides a method of applying amulti-layer thermal barrier coating to a superalloy article comprisingthe steps of:

applying a layer of a platinum-group metal to the superalloy article,

heat treating the article to diffuse the platinum-group metal into thesuperalloy article and thereby create a platinum-group metal enrichedouter layer on the superalloy article, the heat treatment being carriedout at a temperature in the range of above 1100° C. to 1200° C.dependent upon the solution heat treatment temperature appropriate forthe superalloy article and for a time sufficient such that theplatinum-group metal enriched outer layer of the superalloy articlepredominantly comprises a platinum enriched gamma phase and a platinumenriched gamma prime phase, and

applying a ceramic layer to the superalloy article.

The heat treatment is carried out for up to six hours, preferably forone hour.

Preferably the method includes creating a thin adherent layer of oxideon the platinum-group metal enriched outer layer of the superalloyarticle, and applying the ceramic layer on the oxide layer.

The method may include applying an aluminum containing alloy coating tothe platinum-group metal enriched outer layer of the superalloy article,creating a thin adherent layer of oxide on the aluminum containing alloycoating, and applying the ceramic layer to the oxide layer. Thealuminum-containing alloy coating may comprise a MCrAlY alloy, where Mis at least one of Ni, Co and Fe.

Preferably the platinum-group metal is applied by electroplating.

Preferably the thickness of the layer of platinum as applied before heattreatment is greater than 3 micrometers. More preferably the thicknessof the layer of platinum as applied before heat treatment is at least 5micrometers. Preferably the thickness of the layer of platinum asapplied before heat treatment is less than 12.5 micrometers.

Preferably the thin adherent layer of oxide is created by heating theplatinum-group metal enriched outer layer in an oxygen containingatmosphere.

Preferably the thin adherent layer of oxide is created by heating thealuminum containing alloy coating in an oxygen containing atmosphere.

Preferably the ceramic layer is applied by electron beam physical vapordeposition.

Preferably the thin adherent layer of oxide is created during theprocess of electron beam physical vapor deposition.

Preferably a controlled amount of hafnium, or yttrium, is applied with,or to, the layer of platinum-group metal. Preferably the hafnium oryttrium is applied by physical vapor deposition, e.g. sputtering, or bychemical vapor deposition. Preferably hafnium up to 0.8 wt % is added,or yttrium up to 0.8 wt % is added.

Preferably the superalloy article comprises more than 4.5 wt % aluminum,less than 1.5 wt % hafnium and less than 1.5 wt % titanium.

The method may include applying an additional layer, for example cobalt,or chromium, to the superalloy article before applying theplatinum-group metal to the superalloy article. Alternatively the methodmay include applying an additional layer, for example cobalt, orchromium, to the platinum-group metal layer before heat treating thearticle to diffuse the platinum into the superalloy article. Preferablythe additional layer is applied by state of the art techniques forexample by physical vapor deposition (PVD), by an electroplating processor by chemical vapor deposition (CVD). Preferably the thickness of theadditional layer as applied before heat treatment is up to 8micrometers.

Preferably the platinum-group metal is platinum.

The present invention also provides a multi-layer thermal barriercoating for a superalloy article comprising a platinum-group metalenriched outer layer on the superalloy article, the outer layer of thesuperalloy predominantly comprising a platinum-group metal enrichedgamma phase matrix and a platinum-group metal enriched gamma primephase, a thin adherent layer of oxide on the platinum-group metalenriched outer layer of the superalloy article, and a ceramic layer onthe oxide layer.

The present invention also provides a multi-layer thermal barriercoating for a superalloy article comprising a platinum-group metalenriched outer layer on the superalloy article, the outer layer of thesuperalloy predominantly comprising a platinum-group metal enrichedgamma phase matrix and a platinum-group metal enriched gamma primephase, an aluminum containing alloy coating on the platinum-group metalenriched outer layer of the superalloy article, a thin adherent layer ofoxide on the aluminum containing alloy coating, and a ceramic layer onthe oxide layer.

Preferably the ceramic layer comprises yttria stabilized zirconia.

Preferably the ceramic layer has a columnar structure.

Preferably the superalloy substrate comprises a nickel based superalloy.

Preferably the superalloy article comprises more than 4.5 wt % aluminum,less than 1.5 wt % hafnium and less than 1.5 wt % titanium.

Preferably the platinum-group metal enriched outer layer compriseshafnium up to 0.8 wt %, or yttrium up to 0.8 wt %.

The platinum-group metal enriched layer may be enriched in cobalt orchromium.

Preferably the platinum-group metal is platinum.

The invention will be more fully described by way of example withreference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional diagrammatic view through a metallic articlehaving a thermal barrier coating produced by a method according to thepresent invention.

FIG. 2 is cross-sectional diagrammatic view through an alternativemetallic article having a thermal barrier coating produced by a methodaccording to the present invention.

FIG. 3 is an enlarged cross-sectional view through the thermal barriercoating in FIG. 2.

FIG. 4 is a bar chart showing the results of tests of relativeperformance of four different coating types produced on differentsuperalloy substrates.

FIG. 5 is an enlarged cross-sectional view through a metallic articlehaving a thermal barrier coating produced by a prior art method.

FIG. 6 is a bar chart showing the results of tests of relativeperformance of four different coating types produced using differentheat treatment temperatures.

FIG. 7 is a bar chart showing the results of tests of relativeperformance of four different coating types produced using differentthicknesses of platinum.

FIG. 8 is a bar chart showing the results of tests of relativeperformance of three different coating types produced using platinumcontaining hafnium, platinum and no platinum.

FIG. 9 is a bar chart comparing the time to spall of coatings accordingto the present invention and a prior art coating for cyclic testing at1135° C., maintaining at 1135° C. and maintaining at 1190° C.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, illustrating a superalloy article 10 provided witha multi-layer thermal barrier coating indicated generally by numeral 12.It is shown in the "as manufactured" condition. The thermal barriercoating 12 comprises a platinum enriched outer layer 14 on the surfaceof the substrate of the superalloy article 10, a MCrAlY alloy bond coatlayer 16 on the platinum enriched layer 14, a thin oxide layer 18 on theMCrAlY alloy bond coat layer 16 and a ceramic layer 20 on the thin oxidelayer 18. The MCrAlY is generally a NiCrAlY, a CoCrAlY, a NiCoCrAlY or aFeCrAlY, as is well known to those skilled in the art.

The superalloy article 10, which forms the substrate for the coating 12,was made of a nickel or cobalt based superalloy.

In this example the MCrAlY bond coat alloy 16 has a nominal compositiongiven in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        ELEMENT   WEIGHT % MINIMUM                                                                             WEIGHT % MAXIMUM                                     ______________________________________                                        Nickel    31.0           33.0                                                 Chromium  29.0           22.0                                                 Aluminum  7.0            9.0                                                  Yttrium   0.35           0.65                                                 Carbon    0.0            0.025                                                Oxygen    0.0            0.050                                                Nitrogen  0.0            0.010                                                Hydrogen  0.0            0.010                                                Other Elements                                                                          0.0            0.5                                                  in total                                                                      Cobalt    Balance                                                             ______________________________________                                    

The alloy specified in Table 1 is available from Praxair SurfaceTechnologies Inc, (formerly Union Carbide Coating Service Corporation),of Indianapolis, U.S.A., under the trade name LCO22.

To produce the coating 12 the following procedure is followed. Afterthorough preparation and cleaning of the surface of the superalloyarticle 10 by grit blasting with fine alumina grit and degreasing, alayer of platinum having a substantially constant thickness of about 8micrometers was applied to the superalloy substrate. The thickness ofthe platinum layer may vary upwards from about 3 micrometers, dependingupon a number of factors, such as substrate and bond coat materialsused, diffusion temperatures and service conditions. The platinum layerwas applied by electroplating, but any other means could be used whichwill achieve a sufficient substantially uniform thickness withoutdetriment to the material's properties.

A diffusion heat treatment step was then effected so as to cause theplatinum layer to combine with the superalloy substrate 10. Thisprovided the platinum enriched outer layer 14 on the superalloysubstrate 10. Diffusion was achieved by heating the superalloy article10 to a temperature of 1190° C. in a vacuum chamber and holding thattemperature for one hour. In performing the invention a range of heattreatment temperatures may be used from 1100° C. to 1200° C. inclusive,according to the solution heat treatment temperature normally used forthe superalloy article 10. In the present example, 1190° C. is higherthan the accepted solution heat treatment temperature for the superalloyarticle, but was utilized as one of a range of diffusion treatments asexplained later. Although different diffusion times may be used, forexample diffusion times up to six hours have been used, it was judgedthat one hour was sufficient at this range of temperatures for theplatinum to be properly combined with the superalloy substrate 10without prematurely aging the superalloy substrate 10. We have foundthat there is no significant benefit from using diffusion times greaterthan about one hour.

The microstructure of the superalloy substrate generally comprises twophases, these being a gamma phase and a gamma prime phase. The gammaprime phase forms a reinforcement in the gamma phase matrix. The heattreatment of the platinum layer on the superalloy substrate causesaluminum in the superalloy substrate to be diffused outwards towards theplatinum layer on the surface of the superalloy substrate. This resultsin the formation of a platinum enriched gamma phase and a platinumenriched gamma prime phase in the outer surface layer of the superalloyarticle. After heat treatment the surface was grit blasted with dryalumina powder of a 120-220 micrometer particle size to remove anydiffusion residues.

The MCrAlY alloy powder mix was thermally sprayed in known manner ontothe platinum enriched layer 14 of the superalloy article 10 by use of aplasma gun in an evacuated chamber to produce the MCrAlY bond coat layer16. Alternatively the MCrAlY may be applied by any other suitable methodfor example electron beam physical vapor deposition (EBPVD).

To ensure bonding of the MCrAlY bond coat layer 16 to the platinumenriched outer layer 14 of the superalloy article 10, the MCrAlY coatedsuperalloy article 10 was diffusion heat treated at 1100° C. for onehour. This produces a three phase alloy microstructure in the MCrAlYalloy bond coat layer 16. The microstructure of the MCrAlY bond coatlayer 16 broadly comprises three phases, these being an alpha phase, abeta phase, and a small amount of an yttrium-rich phase. The alpha phasecomprises a solid solution of nickel, cobalt, chromium, yttrium andaluminum. The beta phase comprises an aluminide of cobalt, nickel andaluminum, with chromium and other metallic elements dissolved in thealuminide up to certain solubility limits.

After removal of diffusion residues by grit blasting and degreasing, alayer of a ceramic 20 consisting of partially stabilized zirconia (inthis case, zirconia containing 8% by weight of yttria) was applied byelectron beam physical vapor deposition (EBPVD). This coating isavailable from Chromalloy Gas Turbine Corporation of Delaware, U.S.A.

For the EBPVD process, the article was first held in a preheatingchamber and preheated to a temperature of about 1000° C. at a pressureof about 10⁻⁵ Torr. It was then immediately transferred to an electronbeam coating chamber, where it continued to be held for coating at 1000°C., at a pressure of 10⁻² to 10⁻³ Torr, in an atmosphere consisting ofargon and oxygen.

It should be noted that some of the free oxygen in the coating chamber'satmosphere results from the dissociation of zirconia as it is evaporatedby the electron beam in the coating chamber. The dissociatedconstituents of the ceramic recombine with each other as the vapor isdeposited on the article. However, unless assisted, this recombinationtends to be incomplete, i.e., the oxygen binds to the zirconium insub-stoichiometric proportions, resulting in a deficiency of oxygen inthe ceramic and free oxygen in the atmosphere of the coating chamber.Recombination of the ceramic in stoichiometric proportions is assistedby providing an excess of oxygen, thereby further adding to the amountsof oxygen in the coating chamber.

The presence of oxygen at an elevated temperature during the EBPVDcoating process made it inevitable that a thin oxide layer 18 formed onthe surface of the MCrAlY bond coat layer 16. The oxide layer 18 wascovered by the ceramic layer 20 and the oxide layer comprises a mixtureof alumina, chromia and other spinels.

EXAMPLE 1

A batch of specimens as illustrated in FIG. 1 were produced using anickel superalloy called MAR-M 002, a trade name of the Martin MariettaCorporation, of Bethesda, Md., U.S.A. Its nominal composition is givenin Table 2 below.

                  TABLE 2                                                         ______________________________________                                        ELEMENT       WEIGHT %                                                        ______________________________________                                        Tungsten      10                                                              Cobalt        10                                                              Chromium      9                                                               Aluminum      5.5                                                             Tantalum      2.5                                                             Titanium      1.5                                                             Hafnium       1.5                                                             Carbon        0.15                                                            Nickel        Balance                                                         ______________________________________                                    

Some specimens were subjected to a standardized adhesion test in whichthe strength of the bond between the ceramic layer and the platinumenriched outer layer on the superalloy substrate was determined. Onaverage it was found that the critical load, beyond which the ceramicwould break away from the superalloy substrate was about 85 Newtons.

The remaining specimens were then subjected to an aging process tosimulate a period of service in the turbine of a gas turbine engine. Theaging processes were 100 hours at 1050° C., 100 hours at 1100° C. and100 hours at 1150° C. On average it was found that the critical load,beyond which the ceramic would break away from the superalloy substratewas about 65 Newtons for aging for 100 hours at 1050° C., 40 Newtons foraging for 100 hours at 1100° C. and 0 Newtons for aging for 100 hours at1150° C.

Referring to FIG. 2, illustrating a superalloy article 20 provided witha multi-layer thermal barrier coating indicated generally by numeral 22.It is shown in the "as manufactured" condition. The thermal barriercoating 22 comprises a platinum enriched outer layer 24 on the surfaceof the substrate of the superalloy article 20, a thin oxide layer 26 onthe platinum enriched layer 24 and a ceramic layer 28 on the thin oxidelayer 26.

The superalloy article 20, which forms the substrate for the coating 22,was made of a nickel or cobalt based superalloy.

To produce the coating 22 the following procedure is followed. Afterthorough preparation and cleaning of the surface of the superalloyarticle 20 by grit blasting with fine alumina grit and degreasing, alayer of platinum having a substantially constant thickness of about 8micrometers was applied to the superalloy substrate. The thickness orthe platinum layer may again vary upwards from about 3 micrometers,depending upon a number of factors, such as substrate, diffusiontemperatures and service conditions. The platinum layer was applied byelectroplating, but any other means could be used which will achieve asufficient substantially uniform thickness without detriment to thematerial's properties.

A diffusion heat treatment step was then effected so as to cause theplatinum layer to combine with the superalloy substrate 20. Thisprovided the platinum enriched outer layer 24 on the superalloysubstrate 20. Diffusion was achieved by heating the superalloy article20 to a temperature of 1150° C. in a vacuum chamber and holding thattemperature for one hour. In performing the invention a range of heattreatment temperatures may be used from 1100° C. to 1200° C. inclusive,according to the solution heat treatment temperature normally used forthe superalloy article 20. Although different diffusion times could beused, for example up to six hours may be used, it was judged that onehour was sufficient at this range of temperatures for the platinum to beproperly combined with the superalloy substrate 20 without prematurelyaging the superalloy substrate 20.

The microstructure of the superalloy substrate 20 generally comprisestwo phases, as seen more clearly in FIG. 3, these being a gamma phasematrix 30 and a gamma prime phase 32 in the gamma phase matrix 30. Thegamma prime phase 32 forming a reinforcement in the gamma phase matrix30. The heat treatment of the platinum layer 34 on the superalloysubstrate 20 causes aluminum in the superalloy substrate 20 to bediffused outwards towards the platinum layer 34 on the surface of thesuperalloy substrate 20. This results in the formation of a platinumenriched gamma phase 36 and a platinum enriched gamma prime phase 38 inthe outer surface layer of the superalloy article 20. The aluminum inthe platinum enriched outer surface layer 24 of the superalloy article20 is available for formation of alumina 26. It is to be noted that theregion 40 of the superalloy article 20 immediately below the platinumenriched outer surface layer 24 does not have any gamma prime phase 32.The heat treatment causes the aluminum in this gamma prime phase to moveto the platinum layer 34 and hence breaks down the gamma prime phase dueto aluminum's greater chemical affinity for platinum.

It is to be noted that some of the regions of platinum enriched gammaprime phase 38 in the platinum enriched outer surface layer 24 havedistinct promontories, or pegs, which have grown inwardly into theregion 40 of the superalloy article 20. It is believed that theseplatinum enriched gamma prime phase pegs 42 grow into the superalloyarticle and draw the aluminum from the gamma prime phase regions in thesuperalloy article 20. Thus it can be seen that the platinum in theplatinum layer only diffuses into the superalloy article 20 in thesedistinct platinum enriched gamma prime pegs 42, rather than as acontinuous band of platinum. The extent of growth of the platinumenriched gamma prime pegs 42 is sensitive to the thickness of theplatinum layer and the diffusion temperature, as discussed later.

The platinum levels in the platinum enriched gamma prime phase 38 andthe platinum enriched gamma phase 36 are about equal, showing that bothof these phases are equally favored.

It is also to be noted that if there is sufficient aluminum in thesuperalloy article a continuous platinum enriched gamma prime phaseforms on a platinum enriched gamma phase matrix containing platinumenriched gamma prime phases. Furthermore there is always a layer ofplatinum enriched gamma phase immediately underneath the alumina layeras platinum enriched gamma prime phase breaks down to the platinumenriched gamma phase when it loses aluminum to form alumina.

To enhance the thermal barrier coating adhesion to the superalloyarticle 20 it is desirable to ensure phase stability within the platinumenriched gamma phase 36 and the platinum enriched gamma prime phase 38.The stability is achieved by appropriate selection of the platinumthickness within the specified heat treatment temperature range of above1100° C. to 1200° C. In addition it is important to ensure that anyphase changes which occur in operation, within a gas turbine engine,result in small volume changes. This is achieved by control of thecomposition of the platinum enriched gamma phase 36 and platinumenriched gamma prime phase 38. The composition of the platinum enrichedgamma and platinum enriched gamma prime phases are balanced, i.e. thecompositions are closely matched, and any changes from the platinumenriched gamma prime phase to the platinum enriched gamma phase onlyresults in small volume changes.

After heat treatment the surface was grit blasted with dry aluminapowder of a 120-220 micrometer particle size to remove any diffusionresidues.

After removal of diffusion residues by grit blasting and degreasing, alayer of a ceramic 28 consisting of partially stabilized zirconia (inthis case, zirconia containing 8% by weight of yttria) was applied byelectron beam physical vapor deposition (EBPVD). This coating isavailable from Chromalloy Gas Turbine Corporation of Del., U.S.A.

For the EBPVD process, the article was first held in a preheatingchamber and preheated to a temperature of about 1000° C. at a pressureof about 10⁻⁵ Torr. It was then immediately transferred to an electronbeam coating chamber, where it continued to be held for coating at 1000°C., at a pressure of 10⁻² to 10⁻³ Torr, in an atmosphere consisting ofargon and oxygen.

The presence of oxygen at an elevated temperature during the EBPVDcoating process made it inevitable that a thin oxide layer 26 formed onthe surface of the platinum enriched outer layer 24 of the superalloyarticle 20 which comprises the platinum enriched gamma phase 36 andplatinum enriched gamma prime phase 38. The oxide layer 26 was coveredby the ceramic layer 28 and the oxide layer comprises alumina.

EXAMPLE 2

A batch of specimens as illustrated in FIG. 2 were produced using anickel superalloy called CMSX-4, a trade name of the Cannon-MuskegonCorporation, of 2875 Lincoln Street, Muskegon, Mich., MI 49443-0506U.S.A. Its nominal composition is given in Table 3 below. The superalloyarticle specimens were coated with 7 micrometers thickness of platinum.

                  TABLE 3                                                         ______________________________________                                        ELEMENT       WEIGHT %                                                        ______________________________________                                        Tungsten      6.4                                                             Cobalt        9.5                                                             Chromium      6.5                                                             Rhenium       3.0                                                             Aluminum      5.6                                                             Tantalum      6.5                                                             Titanium      1.0                                                             Hafnium       0.1                                                             Molybdenum    0.6                                                             Carbon        0.006                                                           Nickel        Balance                                                         ______________________________________                                    

Some specimens were subjected to a standardized adhesion test in whichthe strength of the bond between the ceramic layer and the platinumenriched outer layer on the superalloy substrate was determined. Onaverage it was found that the critical load, beyond which the ceramicwould break away from the superalloy substrate was about 30 Newtons.

The remaining specimens were then subjected to an aging process tosimulate a period of service in the turbine of a gas turbine engine. Theaging processes were 25 hours at 1100° C., 25 hours at 1150° C. and 25hours at 1170° C. On average it was found that the critical load, beyondwhich the ceramic would break away from the superalloy substrate wasabout 10 Newtons for aging for 25 hours at all three temperaturestested.

EXAMPLE 3

A batch of specimens as illustrated in FIG. 2 were produced using anickel superalloy called CMSX-10, a trade name of the Cannon-MuskegonCorporation, of 2875 Lincoln Street, Muskegon, Mich., MI 49443-0506U.S.A. Its nominal composition is given in Table 4 below. The superalloyarticle specimens were coated with 7 micrometers thickness of platinum.

                  TABLE 4                                                         ______________________________________                                        ELEMENT       WEIGHT %                                                        ______________________________________                                        Tungsten      5.5                                                             Cobalt        3.3                                                             Chromium      2.2                                                             Rhenium       6.2                                                             Aluminum      5.8                                                             Tantalum      8.3                                                             Titanium      0.2                                                             Molybdenum    0.4                                                             Niobium       0.1                                                             Nickel        Balance                                                         ______________________________________                                    

Some specimens were subjected to a standardized adhesion test in whichthe strength of the bond between the ceramic layer and the platinumenriched outer layer on the superalloy substrate was determined. Onaverage it was found that the critical load, beyond which the ceramicwould break away from the superalloy substrate was about 25 Newtons.

The remaining specimens were then subjected to an aging process tosimulate a period of service in the turbine of a gas turbine engine. Theaging processes were 25 hours at 1100° C., 25 hours at 1150° C. and 25hours at 1170° C. On average it was found that the critical load, beyondwhich the ceramic would break away from the superalloy substrate wasabout 10 Newtons for aging for 25 hours at all three temperaturestested.

EXAMPLE 4

A batch of specimens as illustrated in FIG. 2 were produced using anickel based superalloy called MAR-M 002, a trade name of the MartinMarietta Corporation, of Bethesda, Md., U.S.A. Its nominal compositionis given in Table 2 above. The superalloy article specimens were coatedwith 7 micrometers thickness of platinum.

Some specimens were subjected to a standardized adhesion test in whichthe strength of the bond between the ceramic layer and the platinumenriched outer layer on the superalloy substrate was determined. Onaverage it was found that the critical load, beyond which the ceramicwould break away from the superalloy substrate was about 20 Newtons.

The remaining specimens were then subjected to an aging process tosimulate a period of service in the turbine of a gas turbine engine. Theaging processes were again 25 hours at 1100° C., 25 hours at 1150° C.and 25 hours at 1170° C. On average it was found that the critical load,beyond which the ceramic would break away from the superalloy substratewas about 0 Newtons for aging for 25 hours at all three temperaturestested.

FIG. 4 shows the comparative adhesion strengths for coatings accordingto Examples 1, 2, 3 and 4 when subjected to a range of aging treatments.

It is seen from Examples 2, 3 and 4 that the application of platinum andthe heat treatment of the platinum plated superalloy article does notalways produce a satisfactory bond between the superalloy article andthe ceramic coating. We believe this is because of the constituents inthe superalloy article. For example the process produces satisfactorybonds for CMSX-4 and CMSX-10 but not for MAR-M 002. This is because theproportions of hafnium and titanium in particular in the MAR-M 002 aretoo high.

It is believed that the platinum in the platinum enriched outer layer 24of the superalloy article aids the formation of alumina 26, which is thekey to the bonding of the ceramic layer 28 to the superalloy article 20.The platinum in the platinum enriched outer layer 24 causes the aluminumto diffuse from the superalloy substrate into the platinum enrichedouter layer 24 of the superalloy article 20. It is also believed thatthe platinum in the platinum enriched outer layer 24 of the superalloyarticle 20 also acts as a barrier to the migration of transition metalelements from the superalloy article 20 into the base of the ceramiclayer 20 thereby preventing the growth of spinels or other mixed oxideswith lattice structures incompatible with alumina. However, the platinumenriched layer 24 is not sufficient to prevent relatively large amountsof titanium and hafnium migrating from the superalloy article 20 to thebase of the ceramic layer 28. This is because the platinum enrichedlayer 24 comprises the platinum enriched gamma phase and platinumenriched gamma prime phase, and hence there isn't a continuous gammaprime phase barrier to the migration of hafnium and titanium. It ispossible for the titanium and hafnium to migrate through the platinumenriched gamma phase matrix. We have found that if the proportion ofhafnium in the superalloy article is below about 1.5 wt % the platinumenriched layer 24 prevents the hafnium from migrating to the ceramiclayer. We have also found that if the proportion of titanium in thesuperalloy article is below about 1.5 wt % the platinum enriched layer24 prevents the titanium from migrating to the ceramic layer.

Furthermore the relationship between the level of aluminum in thesuperalloy article and the levels of hafnium and titanium is critical.If the ratio of aluminum to hafnium and/or titanium is below about 3 theplatinum enriched layer 24 does not prevent the hafnium and/or titaniummigrating to the ceramic layer.

If the titanium is present in the superalloy as a carbide, for exampletitanium carbide or titanium/tantalum carbide, it is not free to migrateto the oxide bonding layer. However if the titanium is not bound up inthe superalloy as a carbide it is free to migrate to the ceramic layer.Thus if there is more than about 1.5 wt % of free titanium in thesuperalloy then the platinum enriched layer 24 does not prevent thetitanium migrating to the ceramic layer. For example the CMSX-4superalloy which has 1.0 wt % titanium, which is all free titanium asthere is very little carbon to form carbides, forms a satisfactorilybonded ceramic layer. It is believed that the platinum forms strongcomplex molecules with free titanium and prevents the titanium migratingto the ceramic layer. We have found by analysis of the oxide layerformed on the platinum enriched outer layer of the CMSX-4 superalloythat the oxide comprises alumina. However, the oxide layer formeddirectly on the CMSX-4 superalloy without a platinum enriched layercomprises a mixture of alumina and titanium/tantalum oxide. Thus it canbe seen that the platinum enriched layer prevents the migration oftitanium to the ceramic layer.

If the level of aluminum in the superalloy article is below about 4.5 wt% there is insufficient aluminum in the superalloy article to bediffused into the platinum enriched layer 24 to form a continuous layerof alumina.

However hafnium is beneficial to the bonding and formation of thealumina in controlled amounts, below a certain level. We prefer to addcontrolled amounts of hafnium and/or yttrium to the platinum layer. Thehafnium and/or yttrium is added to the platinum layer by physical vapordeposition (PVD), or chemical vapor deposition (CVD). The hafnium isadded to the platinum layer up to a level of 0.8 wt % and the yttrium isadded to the platinum up to a level of 0.8 wt %.

In another variation of the invention a layer of cobalt, or chromium, isapplied onto the superalloy article by electroplating. Subsequently alayer of platinum is applied onto the cobalt, or chromium, layer byelectroplating. Thereafter the cobalt, or chromium, and platinum layerson the superalloy article are heat treated at a temperature in the rangeof 1100° C. to 1200° C. to form a platinum enriched outer layer on thesuperalloy article. This again forms platinum enriched gamma prime andplatinum enriched gamma phases. The cobalt, or chromium, also goes intothe gamma and gamma prime phases and form complexes. The cobalt, orchromium, also enhance the formation of alumina. After the heattreatment step the ceramic layer is again deposited upon an oxide layerwhich forms on the platinum enriched layer before or during the electronbeam physical vapor deposition of the ceramic layer. The cobalt, orchromium, is deposited to a thickness up to 5 micrometers. Alternativelyit is possible to firstly apply the platinum onto the superalloy articleand then to deposit the cobalt, or chromium. However, the formerprocedure is the preferred process.

The attributes of a good bond coating for good adhesion of a thermalbarrier coating are that the bond coating should have the ability toprevent, or reduce, the migration of transition metal elements to theceramic thermal barrier coating. The migration of transition metalelements is preferably blocked by a continuous layer in the bond coatingor slowed by formation of stable compounds by a layer in the bondcoating. This attribute enables the resulting thermally grown oxideformed on the bond coating to be very pure alumina. The bond coatingshould be stable to aging at high temperatures so that it stillprevents, or reduces, the migration of the transition metal elements toensure any further growth of the oxide on the bond coating is byformation of alumina. These attributes are facilitated by the formationof a stable layer close to the thermally grown oxide interface betweenthe bond coating and the ceramic thermal barrier coating.

The advantages of using the ceramic layer directly upon a platinumenriched outer layer of a superalloy article, rather than using ceramicupon a MCrAlY bond coat, ceramic upon a platinum enriched outer layer ofa MCrAlY bond coat is that it has reduced weight, making it moreacceptable for aero gas turbine engines, and it is cheaper tomanufacture. Additionally the coating has better aerothermalperformance. However, the disadvantages are that it does not have ashigh a temperature capability, nor as good a bond with the ceramic, asthe ceramic upon the MCrAlY bond coat or the ceramic upon the platinumenriched outer layer of the MCrAlY bond coat. Nevertheless thetemperature capability of the thermal barrier coating is adequate fortemperatures up to about 1170° C.

EXAMPLE 5

A batch of specimens were produced using a nickel superalloy calledCMSX-4, a trade name of the Cannon-Muskegon Corporation, of 2875 LincolnStreet, Muskegon, Mich., MI 49443-0506 U.S.A. Its nominal composition isgiven in Table 3 above. Some of the superalloy article specimens werecoated with different thicknesses of platinum, these were 3, 7, 12.5 and17.5 micrometers thickness of platinum to determine the effect of theplatinum thickness on the durability of the thermal barrier coating.These specimens were then heat treated at 1150° C. Some of thesuperalloy article specimens were coated with 7 micrometers thickness ofplatinum, and these specimens were heat treated at differenttemperatures to determine the effects of the heat treatment temperature,these were 1000° C., 1100° C., 1150° C. and 1190° on the durability ofthe thermal barrier coating. Some of the specimens as mentioned abovewere platinum plated and heat treated but the ceramic was not applied.An X-ray diffraction analysis of the outer surface of the superalloyarticles after platinum plating and heat treatment was performed.

The X-ray diffraction analysis of the outer surface layer is able topenetrate to a depth of 3-5 micrometers from the surface of thesuperalloy article.

The analysis of the specimen with a 7 micrometer thickness of platinumapplied and which was heat treated at 1000° C., which corresponds to theprior art method taught in U.S. Pat. No. 5,427,866, revealed thatplatinum enriched gamma and platinum enriched gamma prime phases wereproduced, but also an unknown phase was detected. The micrographexamination confirmed that a structure as shown in FIG. 3 was notproduced, but rather had a structure shown in FIG. 5, in which platinumenriched gamma and platinum enriched gamma prime phases are formed withthe unknown phase directly on the platinum enriched gamma and platinumenriched gamma prime phases at the outer surface. The unknown phase hasreduced levels of aluminum, only 3.3 wt %, compared to the platinumenriched gamma and platinum enriched gamma prime phases. It is believedthat this unknown phase will not have enough aluminum to form alumina tobond the ceramic onto the superalloy substrate.

The microstructure of the superalloy substrate 50 generally comprisestwo phases, as seen more clearly in FIG. 5, these being a gamma phasematrix 62 and a gamma prime phase 64 in the gamma prime phase matrix 62.The gamma prime phase 64 forming a reinforcement in the gamma phasematrix 62. The heat treatment of the 7 micrometer thick platinum layer66 on the superalloy substrate 50 causes aluminum in the superalloysubstrate 50 to be diffused outwards towards the platinum layer 66 onthe surface of the superalloy substrate 50. This results in theformation of a platinum enriched gamma phase 68 and a platinum enrichedgamma prime phase 70 and an unknown phase 56 on the phases 68 and 70 onthe outer surface layer of the superalloy article 50. The aluminum inthe unknown phase 56 on the outer surface layer of the superalloyarticle 50 is available for formation of alumina 58 to bond to theceramic layer 60.

In operation the structure shown in FIG. 5 is unstable because theoperating temperatures in the gas turbine engine will cause phasechanges to occur underneath the alumina and ceramic. In particular thereis a significant change in volume if the unknown phase changes to aplatinum enriched gamma phase or platinum enriched gamma prime phase,due to the fact that the size of the crystal structures are so differentbetween the platinum enriched gamma phase, or platinum enriched gammaprime phase, and the unknown phase. This will make the use of thestructure shown in FIG. 5 unsuitable for bonding a ceramic layer to thesuperalloy article, because these volume changes associated with phasechanges will result in the ceramic layer becoming debonded.

The analysis of the specimen with a 7 micrometer thickness of platinumapplied and which was heat treated at 1100° C. revealed that platinumenriched gamma and platinum enriched gamma prime phases were produced,and a micrograph examination confirmed that a structure as shown in FIG.3 is produced. The compositions of the platinum enriched gamma phase andplatinum enriched gamma prime phase are substantially the same as thosefor heat treatment at 1150° C.

The analysis of the specimen with a 7 micrometer thickness of platinumapplied and which was heat treated at 1150° C. revealed that platinumenriched gamma and platinum enriched gamma prime phases were produced,and a micrograph examination confirmed that a structure as shown in FIG.3 is produced. The composition of one of the platinum enriched gammaprime phases is about 53.7 wt % Pt, 29.5 wt % Ni, 2.5 wt % Ta, 1.0 wt %Ti, 4.8 wt % Al, 0.35 wt % Re, 1.6 wt % W, 0.2 wt % Mo, 3.6 wt % Co and3.0 wt % Cr. The composition of one of the platinum enriched gammaphases is 48.6 wt % Pt, 29.7 wt % Ni, 0.8 wt % Ta, 0.3 wt % Ti, 2.75 wt% Al, 1.2 wt % Re, 2.6 wt % W, 0.5 wt % Mo, 6.7 wt % Co and 6.9 wt % Cr.The analysis of the specimen with a 7 micrometer thickness of platinumapplied and which was heat treated at 1190° C. revealed that platinumenriched gamma and platinum enriched gamma prime phases were produced,and a micrograph examination confirmed that a structure as shown in FIG.3 is produced. The compositions of the platinum enriched gamma phase andplatinum enriched gamma prime phase are substantially the same as thosefor heat treatment at 1150° C.

The analysis of the specimen with a 3 micrometer thickness of platinumapplied and which was heat treated at 1150° C. revealed that platinumenriched gamma and platinum enriched gamma prime phases were produced,and a micrograph examination confirmed that a structure as shown in FIG.3 is produced. However, it was found that there was not as much platinumenrichment as for the specimen with a 7 micrometer platinum layer andthus it forms platinum enriched gamma and platinum enriched gamma primephases with different compositions. This is not a continuous platinumenriched outer surface layer.

The analysis of the specimen with a 12.5 micrometer thickness ofplatinum applied and which was heat treated at 1150° C. revealed thatplatinum enriched gamma and platinum enriched gamma prime phases wereproduced, but also an additional unknown phase was detected. Themicrograph examination confirmed that a structure as shown in FIG. 3 wasnot produced, but rather had a structure shown in FIG. 5, in whichplatinum enriched gamma and platinum enriched gamma prime phases areformed with an unknown phase directly on the platinum enriched gamma andplatinum enriched gamma prime phases at the outer surface. The unknownphase has reduced levels of aluminum, only 3.3 wt %, compared to theplatinum enriched gamma and platinum enriched gamma prime phases. It isbelieved that this unknown phase will not have enough aluminum to formessentially pure alumina to bond the ceramic onto the superalloysubstrate.

The analysis of the specimen with a 17.5 micrometer thickness ofplatinum applied and which was heat treated at 1150° C. revealed thesame unknown phase as for the 12.5 micrometer thickness was produced.The micrograph examination confirmed that this unknown phase wasproduced but also platinum enriched gamma and platinum enriched gammaprime phases were produced, and that the structure is as shown in FIG.5, in which platinum enriched gamma and platinum enriched gamma primephases are formed with the unknown phase directly on the platinumenriched gamma and platinum enriched gamma prime phases at the outersurface. The thickness of the unknown phase is much thicker and hencethe X-rays cannot penetrate to the platinum enriched gamma and platinumenriched gamma prime phases. The unknown phase has reduced levels ofaluminum, only 3.3 wt %, compared to the platinum enriched gamma andplatinum enriched gamma prime phases. It is believed that this unknownphase will not have enough aluminum to form essentially pure alumina tobond the ceramic onto the superalloy substrate.

The unknown phase is believed to be some form of ordered phase ofplatinum, nickel and aluminum. The composition of the unknown phase hasbeen determined to be 79 wt % Pt, 12.6 wt % Ni, 3.0 wt % Al, 0.11 wt %Ti, 2.3 wt % Co, 2.7 wt % Cr and trace levels of Re, W, Mo and Ta.

The differences between the structures for the 7 micrometer thickness ofplatinum at the temperatures of 1100 ° C., 1150° C. and 1190° C. is thatthe platinum enriched gamma prime pegs become coarser with increasingtemperature.

Some of these specimens were subjected to a standardized adhesion testin which the strength of the bond between the ceramic layer and theplatinum enriched outer layer on the superalloy substrate was determinedand the results are shown in FIGS. 6 and 7.

The 7 micrometer thickness of platinum, which had been heat treated at1000° C., had a critical load of 10 Newtons in the as processedcondition without any aging. The 7 micrometer thickness of platinum,which had been heat treated at 1100° C., had a critical load of 15Newtons in the as processed condition without any aging. The 7micrometer thickness of platinum, which had been heat treated at 1150°C., had a critical load of 25 Newtons in the as processed conditionwithout any aging. The 7 micrometer thickness of platinum, which hadbeen heat treated at 1190° C., had a critical load of 10 Newtons in theas processed condition without any aging.

The 3 micrometer thickness of platinum, which had been heat treated at1150° C., had a critical load of 10 Newtons in the as processedcondition without any aging. The 12.5 micrometer thickness of platinum,which had been heat treated at 1150° C., had a critical load of 35Newtons in the as processed condition without any aging. The 17.5micrometer thickness of platinum, which had been heat treated at 1150°C., had a critical load of 25 Newtons in the as processed conditionwithout any aging.

The remaining specimens were then subjected to an aging process tosimulate a period of service in the turbine of a gas turbine engine. Theaging processes were 25 hours at 1100° C., 25 hours at 1150° C., 25hours at 1170° C., 25 hours at 1190° C. and 25 hours at 1210° C. andthen the relative adhesion strengths of the different bond coats weredetermined. If the coating failed in the mount of the adhesion strengthtester, then it was assigned a strength of 5 Newtons.

The 7 micrometer thickness of platinum, which had been heat treated at1000° C., had a critical load of 5 Newtons at 1100° C., 10 Newtons at1150° C., but had a critical load of 5 Newtons at 1170° C., 1190° C. and1210° C. The 7 micrometer thickness of platinum, which had been heattreated at 1100° C., had a critical load of 10 Newtons at temperaturesof 1100° C., 1150° C. and 1190° C., and had a critical load of 5 Newtonsat 1170° C. and 1210° C. The 7 micrometer thickness of platinum, whichhad been heat treated at 1150° C., had a critical load of 10 Newtons attemperatures up to 1190° C., and had a critical load of 5 Newtons at1210° C. The 7 micrometer thickness of platinum, which had been heattreated at 1190° C., had a critical load of 10 Newtons at temperaturesof 1100° C. and 1150° C., and had a critical load of 5 Newtons at 1170°C., 1190° C. and 1210° C. The results of the effect on adhesion strengthof different heat treatment temperatures, on a 7 micrometer thickplatinum layer, are shown more clearly in FIG. 6.

The 3 micrometer thickness of platinum, which had been heat treated at1150° C., had a critical load of 5 Newtons at 1100° C. and 1150° C., buthad a critical load of 0 Newtons at 1170° C., 1190° C. and 1210° C.,because the coating spalled. The 12.5 micrometer thickness of platinum,which had been heat treated at 1150° C., had a critical load of 10Newtons at 1100° C., 1150° C. and 1190° C., and had a critical load of 5Newtons at 1170° C. and 1210° C. The 17.5 micrometer thickness ofplatinum, which had been heat treated at 1150° C., had a critical loadof 10 Newtons at temperatures up to 1150° C., and had a critical load of5 Newtons at 1170° C., 1190° C. and 1210° C. The results of the effecton adhesion strength of different thicknesses of the platinum layer, ata heat treatment temperature of 1150° C., are shown more clearly in FIG.7.

Thus it can be seen from the results of the testing that the heattreatment at 1100° C. to 1200° C. for a 7 micrometer thick layer ofplatinum produces better results than for the heat treatment at 1000° C.for a 7 micrometer thick layer of platinum, and in fact there is up to a40° C. temperature capability improvement before failure occurs.Thickness of platinum equal to, or less than, 3 micrometers does notproduce acceptable ceramic thermal barrier coating adhesion. It can beseen that there is some inconsistency in the adhesion strength for the 7micrometer thick platinum layer heat treated at 1100° C. for one hourand for the 12.5 micrometer thick platinum layer heat treated at 1150°C. for one hour. The 12.5 micrometer thick platinum layer may be heattreated at 1150° C. for a longer time period to ensure that all theplatinum combines to form the platinum enriched gamma and platinumenriched gamma prime phases and ensure that none of the unknown phase isformed. It may be possible again to use heat treatment for longer timeperiods for the 17.5 micrometer thick platinum layer, but this does nothave any benefits over the thinner platinum layers and is moreexpensive.

The elements hafnium, titanium and tantalum tend to favor the platinumenriched gamma prime phase. It is postulated that when the levels ofthese transition metal elements in the platinum enriched gamma primephase reaches a critical level there is a reduction in the adhesion ofthe thermal barrier coating. It is theorized that this may be due to thelowering of the gamma prime solvus temperature or any benefits due tothe platinum enrichment are reduced when large concentrations of thesetransition metal elements are present, i.e. the platinum enriched gammaprime phase releases more of the transition metal elements withincreases in concentration of the transition metal elements and thesereleased transition metal elements may effect the oxide layer.

The platinum enriched gamma and platinum enriched gamma prime phasesgrow by drawing elements into their respective phases, and partitioningoccurs in well known ratios. For example in the platinum enriched gammaprime phase the ratio is Ni₃ X or Pt₃ X, where X is Al, Ti, Ta, Hf, Cretc, but there are no restrictions for composition of the platinumenriched gamma phase.

It is postulated that because the transition metal elements are favoredin the platinum enriched gamma prime phase, then the titanium, tantalum,hafnium, etc. are fixed in these phases. In the platinum enriched gammaphase the stability of the titanium, tantalum, hafnium etc is reducedand therefore these transition metal elements are more mobile in thesephases.

We have observed that the outer surface layer of the superalloysubstrate article shown in FIG. 5, generally comprises the unknown phasewhich we have termed the "O" phase, but may in some instances be aplatinum enriched gamma phase. If the outer surface layer is "O" phasethen it tends to have low titanium and tantalum levels because thesetransition metal elements are tied up in the platinum enriched gammaprime phase. Therefore the oxide which forms on the "O" phase isrelatively pure alumina. The "O" phase has a higher level of platinumwhich, it is postulated, compensates to some degree for the lower levelof aluminum when considering alumina formation. If the outer surfacelayer is platinum enriched gamma phase, then the titanium levels areabout the same as for the "O" phase although the tantalum levels arehigher than for the "O" phase. The level of aluminum in the platinumenriched gamma phase is less than that in the "O" phase, but it ispostulated that the additional chromium present relative to the "O"phase will aid formation of alumina.

The platinum enriched gamma prime phase has relatively high levels oftitanium and tantalum but this is balanced by the greater stability ofthe platinum enriched gamma prime phase, which it is postulatedchemically ties up the titanium and tantalum, etc. and prevents themhaving an influence on the adhesion of the ceramic thermal barriercoating.

In all of these cases it is theorized that the platinum suppresses theformation of lesser protective oxide scales by either making theformation of alumina more favorable by increasing the alumina activityor by reducing the movement of transition metal elements through thebond coating by forming strong compounds or by some chemical influenceon the transition metal elements.

EXAMPLE 6

A batch of specimens as illustrated in FIG. 2 were produced using anickel based superalloy called CMSX-4, a trade name of theCannon-Muskegon Corporation, of 2875 Lincoln Street, Muskegon, Mich., MI49443-0506 U.S.A. Its nominal composition is given in Table 3 above. Thesuperalloy article specimens were coated with 5 micrometers thickness ofplatinum alloy containing hafnium by sputtering. The hafnium was presentup to about 0.8 wt %, preferably up to 0.5 wt %. Thereafter the hafniumcontaining platinum layer was heat treated at 1100° C. to 1200° C., inthis example 1150° C., for up to six hours, preferably for one hour todiffuse the hafnium and platinum to form platinum enriched gamma andplatinum enriched gamma prime phases, both of which contain hafnium. Thehafnium content of the specimen was 0.75 wt % and this was compared to aspecimen with 5 micrometer thickness of platinum without hafnium and toa specimen without any platinum.

Some of these specimens were subjected to a standardized adhesion testin which the strength of the bond between the ceramic layer and theplatinum enriched outer layer on the superalloy substrate wasdetermined.

The specimen having 5 micrometer thickness of platinum containinghafnium had a critical load of 25 Newtons in the as processed, withoutaging, condition. The specimen having only 5 micrometer thickness ofplatinum had a critical load of 15 Newtons in the as processedcondition. The specimen without any platinum, i.e. the ceramic layer isbonded directly to an oxide layer on the superalloy substrate, had acritical load of 15 Newtons in the as processed condition.

The remaining specimens were then subjected to an aging process tosimulate a period of service in the turbine of a gas turbine engine. Theaging processes were 25 hours at 1100° C., 25 hours at 1150° C. and 25hours at 1170° C., and then the relative adhesion strengths of thedifferent bond coats were determined.

The specimens with 5 micrometer thickness of platinum had a criticalload of 0 Newtons for aging for 25 hours at 1100° C., 1150° C. and 1170°C. The specimens with 5 micrometer thickness of platinum with thehafnium had a critical load of 5 Newtons for aging for 25 hours at 1100° C. and 1150° C., but had a critical load of 0 Newtons for aging for 25hours at 1170° C. The specimens without any platinum had a critical loadof 0 Newtons for aging for 25 hours at 1100° C., 1150° C., and 1170° C.These results are shown in FIG. 8.

Thus it can be seen that the addition of hafnium to the platinum isbeneficial in that it increases the temperature capability of thethermal barrier coating. It is clear that this would be improved byusing 7 micrometer thickness of platinum.

It is also possible to apply a layer of cobalt, or chromium, eitherbetween the superalloy substrate and the platinum-group metal or on topof the platinum-group metal. The additional layers are generally up to 8micrometer in thickness typically 5 to 8 micrometer in thickness. Theadditional layers may be applied by PVD, CVD or by an electroplatingprocess.

The use of an additional cobalt layer would be beneficial because cobaltimproves the adhesion of the oxide scale on the platinum enriched gammaand platinum enriched gamma prime containing layer. The use of anadditional chromium layer would be beneficial because it aids oxidebonding by increasing the aluminum activity and will also improve theoxidation behaviour of the platinum enriched gamma and platinum enrichedgamma prime phases.

EXAMPLE 7

A batch of specimens as illustrated in FIG. 2 were produced using anickel based superalloy called CMSX-4, a trade name of theCannon-Muskegon Corporation, of 2875 Lincoln Street, Muskegon, Mich.,MI49442-0506, USA. Its composition is given in Table 3 above. Some ofthe superalloy article specimens were coated with 10 micrometersthickness of platinum, were then heat treated at 1100° C. for one hour,were then coated with 7 micrometer thickness of cobalt and heat treatedat 1150° C. for one hour. These specimens were then coated with ceramicby electron beam physical vapor deposition. Some of the superalloyarticle specimens were coated with either 2.5 or 7 micrometers thicknessof cobalt, were then heat treated at either 1000° C. or 1100° C. for onehour, were then coated with 10 micrometer thickness of platinum and werethen heat treated at 1100° C. or 1150° C. for one hour. These specimenswere then coated with ceramic by electron beam physical vapordeposition.

The best results were obtained from the combination of 7 micrometerthickness of cobalt diffused at 1000° C. or 1100° C. with 10 micrometerthickness of platinum diffused at 1150° C. It is theorized that theouter layer of the bond coating contains a top layer of platinumenriched gamma phase and/or a cobalt platinum phase, while an underlayer of the bond coating contains platinum enriched gamma and platinumenriched gamma prime phases, with the transition metal elements beingmaintained in the platinum enriched gamma prime phases. The X-rayanalysis of the under layer to identify the phases is not possible dueto the depth of this layer form the surface.

The composition of the phases for the top and under layers of the bondcoating for the 7 micrometer thickness of cobalt diffused at 1100° C.and 10 micrometer thickness of platinum diffused at 1150° C. are asfollows. The top layer has a first phase with a composition of 76.9 wt %Pt, 10.3 wt % Ni, 7 wt % Co, 3.4 wt % Al, 2.3 wt % Cr, 0.12 wt % Ti, and0.03 wt % Re. The top layer has a second phase with a composition of71.4 wt % Pt, 13.1 wt % Ni, 11 wt % Co, 3.3 wt % Cr, 1.06 wt % Al, and0.09 wt % Ti. The under layer platinum enriched gamma prime phase has acomposition of 72.4 wt % Pt, 12.7 wt % Ni, 5.1 wt % Ta, 3.9 wt % Co, 3wt % Al, 1.1 wt % Cr, 0.9 wt % Ti, 0.77 wt % W and 0.07 wt % Re. Theunder layer platinum enriched gamma phase has a composition of 65 wt %Pt, 16.7 wt % Ni, 11.9 wt % Co, 3.6 wt % Cr, 1.44 wt % Al, 0.5 wt % Ta,0.45 wt % W, 0.27 wt % Ti, 0.15 wt % Re and 0.07 wt % Mo.

In contrast the 2.5 micrometer thickness of cobalt heat treated at 1100°C. followed by 10 micrometer thickness of platinum heat treated at 1150°C. shows higher levels of titanium in the top layer. In cyclic testingat 1135° C., this coating failed after 140 cycles.

FIG. 9 shows the testing for the 7 micrometer thickness of cobalt heattreated at 1100° C. and 10 micrometer thickness of platinum heat treatedat 1150° C. It is apparent from FIG. 9 that this combination of cobaltand platinum layers produced a bond coating which has a better cycliclife at 1135° C. than a single platinum layer, a double platinum layeror a prior art platinum aluminide bond coating produced by platinumplating and aluminizing at 800° C. to 950° C.

EXAMPLE 8

A batch of specimens were produced using a nickel superalloy calledCMSX-4, which has a composition given in Table 3 above. All of thesuperalloy article specimens were coated with 7 micrometer thickness ofplatinum and were heat treated at 1150° C. for one hour. Some of thesuperalloy article specimens were then coated with 5 micrometerthickness of platinum and were heat treated at 900° C. for one hour.Some of the superalloy article specimens were coated with 5 micrometerthickness of platinum and were heat treated at 1000° C. for one hour.Finally some of the superalloy article specimens were then coated with 5micrometer thickness of platinum and were heat treated at 1100° C. forone hour. To all of these superalloy article specimens a ceramic thermalbarrier coating was deposited by electron beam physical vapordeposition.

The intention of these examples was to produce a stable platinumenriched gamma prime phase and platinum enriched gamma phase under layerwith the first application of platinum and its heat treatment at 1150°C. so as to maintain any titanium or other transition metal elements inthis under layer, and to produce a top layer with lower transition metalelement levels with the second application of platinum and its heattreatment at the different temperatures of 900° C., 1000° C. and 1100°C. The titanium and any other transition metal elements would bemaintained in the platinum enriched gamma prime and platinum enrichedgamma phases which are more stable than the gamma prime phase in theCMSX-4 superalloy article.

The analysis of the superalloy article specimens with the secondplatinum layer heat treated a 900° C. showed a top layer containingplatinum enriched gamma phases and "O" phases. The "O" phase is a phasewhich is iso-structural with germanium nickel platinum (Ge Ni Pt₂), thishas an orthorhombic structure distorted cubic. The composition ofgermanium nickel platinum (Ge Ni Pt₂) in atomic percentage is 50 at %Pt, 25 at % Ni and 25 at % Ge, and this compares to a 15 micrometerthickness of platinum diffused at 1150° C. to form an "O" phasecomposition in atomic percentage of 49 at % Pt, 26 at % Ni, 14 at % Al,6 at % Cr, 5 at % Co.

The analysis of the specimens with a second platinum layer heat treatedat 1000° C. and 1100° C. indicates that the amount of titanium in thetop layer increases with increasing heat treatment temperature. Thephases formed in the top layer are platinum enriched gamma prime and "O"phases for heat treatment at 1000° C. and platinum enriched gamma andplatinum enriched gamma prime phases for heat treatment at 1100° C.

Isothermal testing of the specimens in Example 8 and those produced inExample 5 showed that the double layer of platinum with the top layerheat treated at 900° C. out performed the single layer of platinum inExample 5, although cyclic testing showed that both had approximatelythe same number of cycles to failure, 100 to 150 cycles, as shown inFIG. 9.

Our cyclic testing of the coatings indicates that the coatings with the"O" phase in a top layer on top of an under layer comprising platinumenriched gamma and platinum enriched gamma prime phases are equally asgood as the single layer comprising platinum enriched gamma and platinumenriched gamma prime phases.

It was originally believed that the "O" phase would not have sufficientaluminum to form alumina. However, the "O" phase has a relatively lowweight percentage of aluminum but nevertheless this is a relatively highatomic percentage of aluminum. Considering the composition of theplatinum enriched gamma, the platinum enriched gamma prime and the "O"phases produced from diffusion of a single layer of platinum in atomicpercentage:

The platinum enriched gamma phase has a composition of 43-47 at % Ni,22-33 at % Pt, 11-13 at % Cr, 7-10 at % Co, 6-9 at % Al, 0.4-0.8 at % Tiand 0.2-0.6 at % Ta. The platinum enriched gamma prime phase has acomposition of 37-45 at % Ni, 25-34 at % Pt, 14-16 at % Al, 4-8 at % Cr,4-5 at % Co, 1-2 at % Ti and 0.4-2 at % Ta. The "O" phase has acomposition of 44-50 at % Pt, 25-28 at % Ni, 13-17 at % Al, 6-8 at % Cr,4-5 at % Co, 0.3-0.5 at % Ti and 0-0.1 at % Ta.

The present invention provides a bond coating which has low levels oftitanium or other transition metal elements, or if the transition metalelements are present then they are prevented from migrating to thealumina layer and ceramic thermal barrier coating by formation of stablecompounds or phases.

The present invention provides a bond coating which slows the migrationof transition metal elements to the ceramic thermal barrier coating andoxide layer, it does not have a continuous layer which blocks themigration of the transition metal elements. The bond coating istherefore not completely effective at preventing the migration oftransition metal elements to the ceramic thermal barrier coating and theoxide layer.

Nevertheless, the oxide formed on the bond coating is very pure alumina,and has very little or no damaging transition metal oxides such asrutile (TiO₂) or (Ti,Ta)O₂.

In the case of a single thin platinum layer, or high heat treatmenttemperature for a single platinum layer, being diffused into thesuperalloy article the resulting single outer layer comprises platinumenriched gamma and platinum enriched gamma prime phases. In the case ofa single thick platinum layer, or low heat treatment temperature for asingle thin platinum layer being diffused into the superalloy articlethe resulting outer layer comprises a platinum enriched gamma, aplatinum enriched gamma prime or an "O" phase top layer on an underlayer which comprises platinum enriched gamma and platinum enrichedgamma prime phases. In the case of two platinum layers being diffusedinto the superalloy article the resulting outer layer comprises a toplayer comprising a combination of two or more of platinum enrichedgamma, platinum enriched gamma prime or "O" phase on an under layerwhich comprises platinum enriched gamma and platinum enriched gammaprime phases. In the case of a combination of cobalt and platinum layersbeing diffused into the superalloy article the resulting outer layercomprises a top layer comprising either a combination of platinumenriched gamma and cobalt platinum phases or platinum enriched gammaprime in a platinum enriched gamma phase or a cobalt platinum phase onan under layer which comprises platinum enriched gamma and platinumenriched gamma prime phases.

The heat treatment may be carried out either in hard vacuum conditionsat a pressure in the range of 10⁻⁴ to 10⁻⁵ Torr or in partial pressureof argon for example at 10⁻² Torr.

We have found from measurement of the crystal structure of the platinumenriched gamma, platinum enriched gamma prime and "O" phase that thevolume of eight platinum enriched gamma crystal cubes is 403 A^(o3),that the volume of eight platinum enriched gamma prime crystal cubes is414 A^(o3) and that the volume of one "O" phase crystal cube is 425A^(o3). Thus it can be seen there is only about a 5% change in volume inchanging from the "O" phase to the platinum enriched gamma phase. Thereis only about a 2.5% change in volume in changing from the platinumenriched gamma prime phase to the platinum enriched gamma phase. Thesesmall changes will not compromise the adhesion of the bond coating.

A major factor at arriving at the benefits of the present invention isthat the heat treatment range enables full diffusion to occur betweenthe platinum-group metal and the superalloy substrate to form theplatinum enriched gamma and platinum enriched gamma prime phases. Thesephases are very similar in composition and any changes of phase betweenthe two does not disturb the overlying ceramic thermal barrier coatingbecause any changes of phase, in operation, do not result in largevolume changes, compared to the prior art in which there is no, or onlypartial diffusion.

Whereas in the above described examples, only platinum was applied tothe superalloy article and heat treated to diffuse into the outer layerof the superalloy article, other platinum-group metals may besubstituted, such as palladium or rhodium.

There is no reason to suppose that the invention may not also besuccessfully applied to cobalt based superalloys.

A further advantage of the present invention is that the heat treatmentabove 1100° C. improves the yield of successfully coated superalloyarticles compared to those heat treated at 1000° C.

If the present invention is applied to turbine blades, or turbine vanes,the precise thickness of the ceramic layer will be decided upon byexperiment and/or calculation and will depend upon the temperature andcorrosive agents to be experienced by the components during operation.For example the ceramic layer will have a thickness up to 300micrometers (300×10⁻⁶ m).

We claim:
 1. A multi-layer thermal barrier coated article, comprising:asuperalloy substrate; an outer layer on the superalloy substrate, theouter layer being enriched in platinum-group metal relative to an innerportion of the article, and the outer layer comprising a platinum-groupmetal enriched gamma phase and a platinum-group metal enriched gammaprime phase; an adherent layer of oxide on the outer layer of thearticle, and a ceramic coating on the adherent oxide layer.
 2. Amulti-layer thermal barrier coated article as claimed in claim 1 whereinthe outer layer of the article comprises a top layer and an underlayer,the top layer comprising at least one of platinum-group metalenriched gamma phase, platinum-group metal enriched gamma prime phaseand an ordered phase of platinum-group metal, nickel and aluminum, theunder layer comprising platinum-group metal enriched gamma phase andplatinum-group metal enriched gamma prime phase, and the adherent layerof oxide is on the top layer.
 3. A thermal barrier coated article asclaimed in claim 2 wherein the ordered phase of platinum-group metal,nickel and aluminum comprises 44-50 at % Pt, 25-28 at % Ni, 13-17 at %Al, 6-8 at % Cr, 4-5 at % Co, 0.3-0.5 at % Ti and 0-0.1 at % Ta.
 4. Amulti-layer thermal barrier coated article as claimed in claim 1 whereinthe outer layer of the article comprises a top layer and an underlayer,the top layer comprising at least one of platinum-group metalenriched gamma phase and cobalt-platinum phase, and the under layercomprising platinum-group metal enriched gamma phase and platinum-groupmetal enriched gamma prime phase, and the adherent layer of oxide is onthe top layer.
 5. A multi-layer thermal barrier coated article asclaimed in claim 4 wherein the top layer comprises platinum enrichedgamma prime phase.
 6. A multi-layer thermal barrier coated article asclaimed in claim 1 wherein the ceramic coating comprises yttriastabilized zirconia.
 7. A multi-layer thermal barrier coated article asclaimed in claim 1 wherein the ceramic coating has a columnar structure.8. A multi-layer thermal barrier coated article as claimed in claim 1wherein the superalloy substrate comprises a nickel based superalloy. 9.A multi-layer thermal barrier coated article as claimed in claim 1wherein the superalloy substrate comprises more than 4.5 wt % aluminum,less than 1.5 wt % hafnium and less than 1.5 wt % titanium.
 10. Amulti-layer thermal barrier coated article as claimed in claim 1 whereinthe outer layer comprises up to 0.8 wt % hafnium, or up to 0.8 wt %yttrium.
 11. A multi-layer thermal barrier coated article as claimed inclaim 1 wherein the outer layer is further enriched in cobalt orchromium.
 12. A multi-layer thermal barrier coated article as claimed inclaim 1 wherein the platinum-group metal is platinum.
 13. A multi-layerthermal barrier coated article as claimed in claim 1 wherein theplatinum-group metal enriched gamma phase comprises 6-9 at % aluminum,and the platinum-group metal enriched gamma prime phase comprises 14-16at % aluminum.
 14. A multi-layer thermal barrier coated article asclaimed in claim 1 wherein the platinum-group metal enriched gamma phasecomprises 43-47 at % Ni, 22-33 at % Pt, 11-13 at % Cr, 7-10 at % Co, 6-9at % Al, 0.4-0.8 at % Ti and 0.2-0.6 at % Ta, and the platinum-groupmetal enriched gamma prime phase comprises 37-45 at % Ni, 25-34 at % Pt,14-16 at % Al, 4-8 at % Cr, 4-5 at % Co, 1-2 at % Ti and 0.4-2 at % Ta.15. A multi-layer thermal barrier coated article as claimed in claim 1wherein the platinum-group metal enriched gamma phase comprises 2.75 wt% aluminum, and the platinum-group metal enriched gamma prime phasecomprises 4.8 wt % aluminum.
 16. A multi-layer thermal barrier coatedarticle as claimed in claim 1 wherein the platinum-group metal enrichedgamma phase comprises 29.7 wt % Ni, 48.6 wt % Pt, 6.9 wt % Cr, 6.7 wt %Co, 2.75 wt % Al, 0.3 wt % Ti, 0.8 wt % Ta, 1.2 wt % Re, 2.6 wt % W and0.5 wt % Mo, and the platinum-group metal enriched gamma prime phasecomprises 29.5 wt % Ni, 53.7 wt % Pt, 3.0 wt % Cr, 3.6 wt % Co, 4.8 wt %Al, 1.0 wt % Ti, 2.5 wt % Ta, 0.35 wt % Re, 1.6 wt % W and 0.2 wt % Mo.17. A multi-layer thermal barrier coated article, comprising;asuperalloy substrate; an outer layer on the article, the outer layerbeing enriched in platinum-group metal relative to an inner portion ofthe article, and the outer layer comprising a platinum-group metalenriched gamma phase and a platinum-group metal enriched gamma primephase; an aluminum-containing alloy coating on the outer layer of thearticle; an adherent layer of oxide on the aluminum-containing alloycoating; and a ceramic coating on the adherent oxide layer.
 18. Amulti-layer thermal barrier coated article as claimed in claim 17wherein the platinum-group metal enriched gamma phase comprises 6-9 at %aluminum, and the platinum-group metal enriched gamma prime phasecomprises 14-16 at % aluminum.
 19. A multi-layer thermal barrier coatedarticle as claimed in claim 17 wherein the platinum-group metal enrichedgamma phase comprises 43-47 at % Ni, 22-33 at % Pt, 11-13 at % Cr, 7-10at % Co, 6-9 at % Al, 0.4-0.8 at % Ti and 0.2-0.6 at % Ta, and theplatinum-group metal enriched gamma prime phase comprises 37-45 at % Ni,25-34 at % Pt, 14-16 at % Al, 4-8 at % Cr, 4-5 at % Co, 1-2 at % Ti and0.4-2 at % Ta.
 20. A multi-layer thermal barrier coated article asclaimed in claim 17 wherein the platinum-group metal enriched gammaphase comprises 2.75 wt % aluminum, and the platinum-group metalenriched gamma prime phase comprises 4.8 wt % aluminum.
 21. Amulti-layer thermal barrier coated article as claimed in claim 17wherein the platinum-group metal enriched gamma phase comprises 29.7 wt% Ni, 48.6 wt % Pt, 6.9 wt % Cr, 6.7 wt % Co, 2.75 wt % Al, 0.3 wt % Ti,0.8 wt % Ta, 1.2 wt % Re, 2.6 wt % W and 0.5 wt % Mo, and theplatinum-group metal enriched gamma prime phase comprises 29.5 wt % Ni,53.7 wt % Pt, 3.0 wt % Cr, 3.6 wt % Co, 4.8 wt % Al, 1.0 wt % Ti, 2.5 wt% Ta, 0.35 wt % Re, 1.6 wt % W and 0.2 wt % Mo.